celebrity writers

Seed dispersal

Flowering plants use various strategies to spread their seeds

Jan 16, 2007

Species that survive are species that reproduce and spread. Plants can't walk, so in order to spread, flowering plants need ways to get their seeds to new locations.

They use a number of different strategies that we can observe just by strolling through a field in summer, fall, or winter.

Many plants use wind or water to carry their seeds, which are attached to fine, fluffy fibers that act as parachutes. Milkweed and thistle are among the plants whose seeds fly through the air. Trees such as willow, cottonwood, and sycamore, which grow along streams or creeks, drop many of their seeds into the water, where the fluff keeps them afloat on the current so it can carry them to new spots.

Hitchhikers have miniscule hooks attached to the seed capsule so that they can grab onto the fur of passing animals or the clothes of people, to be scratched or pulled off later. Burdock burrs, in fact, were the inspiration for the invention of Velcro.

Then there are the leapers, seeds that bounce or jump away from the parent plant. Jewelweed and witch hazel have seed cases that are so constructed that when they dry out, the slightest touch causes the case to burst and project the mature seeds through the air. Black walnuts have spongy hulls so that they bounce when they hit the ground and roll away from the tree.

What we think of as fruits are actually seeds surrounded by sweet pulp that animals like to eat. Animals often take their food to a safer location to feed, and the pit is discarded after the animal consumes the pulp. If the seeds are small, they are swallowed whole and then excreted, usually far from the parent plant. Many acorns buried by squirrels are never retrieved. These acorns eventually sprout, and some grow into trees.

Species that are able to compete easily with other plants often do not have elaborate dispersal strategies but simply bear their seeds in cases that drop the seeds nearby when brushed by animals or wind. Garlic mustard, for instance, may form large, dense stands of plants. Other species don’t bother much with seeds but spread through rhizomes, or underground stems, such as wild ginger and day lily. Runners are an aboveground variation seen in strawberries.

The abundance of a species depends on many factors, but its dispersal method is often key to survival. In winter, plant skeletons offer evidence of seed dispersal.

The copyright of the article Seed dispersal in Botany is owned by Violet Snow. Permission to republish Seed dispersal in print or online must be granted by the author in writing.

Comments

Jan 16, 2007 11:11 PM

Jennifer W. Miner :

Interesting - thank you!

I just read in the Science Section of today's NY Times that the giant stinky flower - whatever it's name is, it smells like rotting flesh and is native to Madagascar - smells that way to attract flies, which in turn act as its pollinator. Pretty clever huh?

Jan 18, 2007 7:22 AM

Violet Snow :

Yes, the Rafflesia is also distinctive because it is so huge. But there is also a plant called carrion flower that is native to the U.S. and has the same type of odor and pollination strategy. Plants will go to amazing lengths to carve out a niche.

Jan 28, 2007 10:14 PM

Barbara Stewart :

It seems the Rafflesia produces fruit that are eaten by small mammals. Is any more known about seed dispersal and how the seeds germinate? As it is a parasitic plant, it probably needs cues from its host to trigger germination. Maybe it it too rare to provide opportunities for study.

Jan 29, 2007 7:52 AM

Violet Snow :

Edward S. Ross, curator emeritus of entomology at the California Academy of Sciences, writes:

"Botanists are uncertain how Rafflesia seeds disperse and get 'planted' in new hosts. One theory postulates that when hooved animals such as deer, wild pigs, and tapirs inadvertently trample mushy, rotting Rafflesia flowers, the seeds adhere to the hairs on their feet. While walking through the forest, their sharp hooves might cut the bark of a buried liana and push seeds into the wound. Other scientists think that tree shrews or ground squirrels gnaw the columns of dead female flowers, and spread the seed in their waste. Rain helps the seeds, which are only slightly larger than those of poppies, percolate into bark crevices. Either way, the process doesn't succeed often, for the plants are quite rare."

After describing bud formation, he comments:

"If left undamaged, the mature buds literally burst into bloom. I have been told that the sudden unfolding of each fleshy, petal-like lobe makes a sucking sound much like a foot pulled from mud."

Lovely description. For more, see:
http://www.calacademy.org/calwild/2003summer/stories/rafflesia.html

4 Comments